This invention broadly relates to a corrosion resistant coating composition comprising a particulate corrosion resistant component, and a glass-forming binder component. This invention also broadly relates to an article comprising a turbine component coated with at least one layer of this composition. This invention further broadly relates to a method for coating the article with at least one layer of this composition.
In an aircraft gas turbine engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot exhaust gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine by impingement against the airfoil section of the turbine blades, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. Thus, there is incentive to raise the combustion gas temperature.
The compressors and turbine of the turbine engine can comprise turbine disks (sometimes termed “turbine rotors”) or turbine shafts, as well as a number of blades mounted to the turbine disks/shafts and extending radially outwardly therefrom into the gas flow path, and rotating. Also included in the turbine engine are rotating, as well as static, seal elements that channel the airflow used for cooling certain components such as turbine blades and vanes. As the maximum operating temperature of the turbine engine increases, the turbine disks/shafts and seal elements are subjected to higher temperatures. As a result, oxidation and corrosion of the disks/shafts and seal elements have become of greater concern.
Metal salts such as alkaline sulfate, sulfites, chlorides, carbonates, oxides, and other corrodant salt deposits resulting from ingested dirt, fly ash, volcanic ash, concrete dust, sand, sea salt, etc. are a major source of the corrosion, but other elements in the bleed gas environment can also accelerate the corrosion. Alkaline sulfate corrosion in the temperature range and atmospheric region of interest results in pitting of the turbine disk/shaft and seal element substrate at temperatures typically starting around 1000° F. (538° C.). This pitting corrosion has been shown to occur on critical turbine disk/shaft and seal elements. The oxidation and corrosion damage can lead to premature removal and replacement of the disks and seal elements unless the damage is reduced or repaired.
Turbine disks/shafts and seal elements for use at the highest operating temperatures are typically made of nickel-base superalloys selected for good elevated temperature mechanical properties such as fatigue resistance. These superalloys have resistance to oxidation and corrosion damage, but that resistance may not be sufficient to protect them at sustained operating temperatures now being reached in gas turbine engines. Disks and other rotor components made from newer generation alloys may also contain lower levels of chromium, and may therefore be more susceptible to corrosion attack.
Corrosion resistant coating compositions have been suggested for use with various gas turbine components. These include aqueous corrosion resistant coating compositions comprising phosphate/chromate binder systems and aluminum/alumina particles. See, for example, U.S. Pat. No. 4,537,632 (Mosser), issued Aug. 27, 1985 and U.S. Pat. No. 4,606,967 (Mosser), issued Aug. 19, 1986 (spheroidal aluminum particles); and U.S. Pat. No. 4,544,408 (Mosser et al), issued Oct. 1, 1985 (dispersible hydrated alumina particles). Corrosion resistant diffusion coatings can also be formed from aluminum or chromium, or from the respective oxides (i.e., alumina or chromia). See, for example, commonly assigned U.S. Pat. No. 5,368,888 (Rigney), issued Nov. 29, 1994 (aluminide diffusion coating); and commonly assigned U.S. Pat. No. 6,283,715 (Nagaraj et al), issued Sep. 4, 2001 (chromium diffusion coating). A number of corrosion-resistant coatings have also been specifically considered for use on turbine disk/shaft and seal elements. See, for example, commonly assigned U.S. Patent Application 2004/0013802 A1 (Ackerman et al), published Jan. 22, 2004 (metal-organic chemical vapor deposition of aluminum, silicon, tantalum, titanium or chromium oxide on turbine disks and seal elements to provide a protective coating).
Another corrosion resistant coating that has been used comprises an alumina pigment in a chromate-phosphate binder having hexavalent chromium (commercially marketed by Sermatech International as SermaFlow® N3000). While such a hexavalent chromium-containing coating is effective at low temperatures, it has a lower coefficient of thermal expansion relative to the underlying metal substrate (e.g. superalloy) so that at the higher temperatures experienced by newer gas turbine engines, this hexavalent chromium-containing coating may spall, even when applied at thicknesses of as thin as 0.5 to 2.5 mils (12.7 to 63.5 microns). In fact, at thicknesses of greater than 1.5 mils (38.1 microns), this coating may delaminate after one thermal cycle at 1300° F. (704° C.). While this delamination problem is most evident on the newer high performance gas turbine engines, this problem may also occur with other gas turbine engines because of the temperature extremes dictated by engine operation.
While these prior corrosion resistant coatings may provide corrosion protection for turbine disk/shaft and seal elements, there remains a need for improved corrosion resistant coatings that address the disadvantages of these prior corrosion resistant coatings, including: (1) possible adverse affects on the fatigue life of the turbine disks/shafts and seal elements, especially when these prior coatings diffuse into the underlying metal substrate; (2) potential coefficient of thermal expansion (CTE) mismatches between the coating and the underlying metal substrate that may make the coating more prone to spalling; and (3) requiring more complicated and expensive processes (e.g., chemical vapor deposition) for applying the corrosion resistant coating to the metal substrate.